For power switching, devices based on a metal-oxide-semiconductor (MOS) structure, an insulated gate bipolar transistor (IGBT) structure and a lightly doped drain metal-oxide-semiconductor (LDMOS) structure are commonly used. Power devices based on the MOS structure are mainly used in domestic units which are operated at a few hundreds volts. Power devices based on IGBT are for high power switching in applications including AC to DC or DC to AC conversion and are designed to sustain high voltages up to several kilovolts. Power devices based on LDMOS are for intermediate power level applications. Current MOS, LDMOS and IGBT devices are manufactured using silicon technology which has been successful in these applications due to significant research and development in the last six decades. However, the performance of these silicon power devices is still limited mainly due to their limited breakdown electric field.
Recently, a new class of semiconductors based on III-nitrides are being developed, where III represents group three metals: Al, Ga and In. Examples of the new class of semiconductors include AlN, GaN, InN and their alloys such as AlGaN, InGaN and AlInN. Some of these new III-nitrides have exceptional electronic properties compared to crystalline silicon. In addition, energy bandgap values of the III-nitrides, specifically that of GaN, AlGaN and AlN are large compared to silicon and gallium arsenide. Because of the large energy bandgaps, devices fabricated using these III-nitrides semiconductors and their mixtures or alloys have breakdown electric fields substantially greater than that of their counter parts: Si and GaAs. For instance, the breakdown electric field for AlGaN is 3.0×106 V/cm which is about 10 times of that for Si and GaAs, the two most important electronic semiconductors in industry; therefore, the III-nitrides can sustain larger voltages with the same device dimensions or thicknesses. It should also be noted that charge carrier mobilities of the III-nitrides are greater than silicon. Furthermore, the critical temperatures of some of the III-nitrides for stable operation are substantially higher than that of GaAs and Si. As a comparison, the critical junction temperature for stable operation is 250° C. for silicon devices, 400° C. for GaAs devices and it is 600° C. for devices based on GaN, AlGaN and InGaN. Combining high breakdown electric field, high charge carrier mobility and high critical temperature for stable operation of the III-nitride devices, it is evident that these devices and circuits are ideal for high power switching and high frequency millimeter wave circuit applications and it is possible for the III-nitrides to replace some of the high frequency applications currently provided by GaAs technology.
However, due to the difference between the III-nitrides materials and the sapphire or SiC substrates used for these devices, there is often a mismatch in their thermal expansion coefficients and their lattices. These differences in thermal expansion coefficients and lattices will induce strain or stresses in the epitaxial III-nitride thin films and the substrate during cooling or heating stages. These strain or stresses may lead to microcracks in the epitaxial III-nitride layers and the electronic properties of the films may be affected due to the presence of the microcracks. It should be noted that these microcracks are often too small to be unequivocally observed under simple optical microscopes with a low magnification.
Although current III-nitride devices are mainly manufactured on sapphire or SiC substrates, both sapphire and SiC substrates are more difficult or expensive to manufacture due to the nature of the compounds and their high melting points. On the contrary, silicon wafers have been developed in the last six decades and the manufacturing technology is more mature. Hence the substrate cost is lower and the supply more abundant. If high quality III-nitride layers and devices can be developed on Si wafers, then high power and high frequency devices may be achieved with reduced manufacturing cost. While vigorous recent efforts have been made on research and development of III-nitride epitaxial growth on silicon wafer and device fabrication, there are still obstacles to be overcome. One of the obstacles is related to the present invention and is resulted from lattice mismatch and thermal expansion coefficient difference between materials. During deposition of III-nitrides, the substrate temperatures are often in the order of 1000° C. After the deposition and during cooling, significant strain or stresses can be induced in the deposited III-nitride epitaxial layers and the substrates. Since the epitaxial layers have much smaller thickness compared to the substrates, microcracks will form in the deposited III-nitride films. The microcracks can degrade the electronic performance and reliability of the fabricated devices and circuits. The present invention provides a structure and a method of manufacturing to minimize the degradation on performance and reliability caused by microcracks in the epitaxial III-nitride films.